Process for pulping



United States Patent Office 2,708,160 Patented May 10, 1955 PROCESS FORPULPING Samuel I. Aronovsky and Elbert C. Lathrop, Peoria, 111.,-

assignors to the United States of America as represented by theSecretary of Agriculture N Drawing. Application August 17, 1949, SerialNo. 110,868

4 Claims. (Cl. 92-6) (Granted under Title 35, U. S. Code (1952), sec.266) i This application is made under the act of March 3,

1883, as amended by the act of April 30, 1928, and the invention hereindescribed, if patented in any country, may be manufactured and used byor for the Government of the United States of America for governmentalpurposes throughout the world without the payment to us of any royaltythereon.

This invention relates to a novel process for manufacturing paper pulpfrom coarse fiber materials. It has among its objects the provision of anovel mechanochemical pulping process which can be conducted atatmospheric pressures at reduced operating and equipment cost, atreduced pulping time and with improved quality and uniformity ofproduct.

Coarse fiber material may be defined for the purpose of this inventionas cellulosic material of a basic fibrous structure which occursnaturally or has been converted into material consisting substantiallyof fiber bundles or groups of fiber bundles which have a relatively thincross section measured perpendicular to the axis of the fiber directionor grain of the material. In the case of woody materials thecross-sectional dimension should be less than or of the same order ofmagnitude as the individual fiber lengths. This limitation on the woodymaterials is necessary in view of the density or compactness of thewoody structure, which in greater thicknesses wouldtend to restrict orresist the facile impregnation of liquids or liquid chemicals,-whenemploying the process of this invention. Examples of coarse fibermaterial in accordance with this invention are such non-woody materialsas straw, flax and hemp tow, cotton, sugar-cane bagasse, cornstalks,jute, reeds, manila and sisal fibers and the like. Included also arecoarse fiber materials produced from wood, such as Asplund fiber,McMillan fiber, coarse groundwood fiber, chopped fine excelsior and thelike.

Another object is the provision of a combined chemical digestion andmechanical defibering process which results in larger yields of pulpwith higher hemicellulose content, requiring less power and equipmentfor beating and refining. Still another object is to provide a strawpulp substantially free from node or rachis material and seeds, andcontaining a minimum of fines.

Pulp from straw, flax and hemp tow, cotton, sugarcane bagasse,cornstalks, jute, reeds, manila and sisal fibers or similar material isusually prepared by pressure pulping methods which involve heating forseveral hours in the chemical pulping liquor at temperaturessubstantially above 100 C. and at corresponding steam pressures. Thesemethods are attended by the many hazards and other disadvantages whichattend the heating of large volumes of material under considerablepressure for extended periods of time. Even though high quality paperpulp'can be made by these methods, they are relatively tedious andcostly due to the necessity for pressure equipment and the great amountof labor and time required. Although it is known that agriculturalresidues such as straw can be pulped at atmospheric pressure, methodsheretofore employed have required relatively large amounts of chemicalsand long cooking or digestion periods.

We have discovered that a large factor in the speed and properconversion of raw stock to pulp is the rate of diffusion of the liquidcontaining the active chemical through the pieces of raw material tosucceeding inner layers or surfaces. Thus the reaction between a pulpingagent and the plant material to be delignified may be considered atopochemical reaction; that is, it takes place mainly at the contactsurfaces or interfaces between the solid plant material and the liquid.When the reaction at this point is completed no further substantialaction takes place until the reaction products have been removed, thusexposing new surfaces. During the early part of the conventionalcooking, such removal may be accomplished by circulation of the liquorpast the more or less stationary plant material in a stationarydigester, or by the difference in motion of the liquor and plantmaterial, relative to each other, in a tumbling or rotating digester.After the first action on the outside surfaces of the solid material,the reaction products from the reacting surfaces have to be removed,fresh chemical has i to be brought to the reacting surfaces. Theseactions are normally brought about by diffusion, which is a relativelyslow process. It is mainly for this reason that the rate of pulping byconventional methods decreases as the cooking progresses.

According to the instant invention a mechano-chemical process forproducing pulp is provided. In this process, an undefibered non-woodyfiber material, such as straw, bagasse, etc. is subjected to the actionof a pulping liquor containing a chemical reagent in solution in aliquid medium, this reagent being capable of digesting the saidnon-woody fiber material by removing lignin and other encrustantstherefrom. The process is carried out at a temperature ranging fromabout to C. and at atmospheric pressure and under these conditions thepulping liquor is essentially non-hydrolytic. Simultaneously with theforegoing chemical action, the individual pieces of the non-woody fibermaterial are subjected to vigorous non-cutting impact blows whilesubmerged in the pulping liquor and while they are in a relatively freestate, these non-cutting impact blows comprising mechanically-inducedcycles of expression and absorption. Each cycle comprises compacting thepieces in an unconfined zone of impact to express liquid, immediatelyremoving said pieces from the zone of impact to a zone of absorption offresh liquid and returning said pieces containing absorbed liquid toanother zone of impact. These blows are continued for from 30 to 60minutes at which time a substantial proportion of liquor has beenremoved from the pieces and they have become broken up into individualfibers producing a pulp having a proportion of hemi-celluloses at leastas great as that contained in the original non-woody fiber material.Where the non-woody starting material contains nodes, rachises, seeds,and similar materials, the pulp produced by the above-described processmay be subjected to a rifiling and screening operation to remove suchnodes, rachises, etc., from the pulp.

- We have utilized our discoveries and have provided a pulping processwhich involves a simultaneous mechanical and chemical action on thestock during the pulping whereby the rate of diffusion is greatlyincreased. We accomplish the increase in diffusion rate by mechanicalimpact upon the liquid-swollen material in a relatively free state whilesubmerged in the liquor. Upon a vigorousblow being struck whilesubmerged in the cooking liquor, the turgid material is compacted,causing some of the liquid to be expressed, as liquid may be squeezedfrom a sponge, and immediately thrown from the zone of impact. Afterleaving the Zone of impact, the material tends to recover its originalvolume and absorb fresh liquid. Thus, by rapid mechanically inducedcycles of expression and absorption, the diffusion of thechemicalcontaining liquid in and out of the material is speeded upgreatly, resulting in astonishingly increased pulping rates, of theorder of at least four times that of conventional pressure pulping.

This rapid increase in rate of dilfusion can be accomplished by variousmechanical means. A rapidly rotating plate fitted with non-cutting vanessubmerged to the liquor, or a high-speed propeller stirrer producesatisfactory results. The impact action upon which our invention isbased is not to be confused with the shearing or tearing action of theconventional pulp beater, jordan or other conventional refining engines.Moreover, prior methods for producing pulp employing a substantialdegree of mechanical action have involved the use of heaters, mixers,paddle wheels, breaker heaters and the like.

In these prior processes, the pieces of material to be pulped areconfined mechanically and cut or torn apart by the opposed action ofrelatively unyielding rubbing, grinding or cutting members or by theinternal friction of a high consistency mass. Such action between twosurfaces, squeezing and grinding the material, disintegrates the nodesand rachises and/or ruptures and cuts the fibers. In our process theparticles are pulped and the cycles of diifusion are induced while in afree floating state in a medium of relatively low consistency, so thatwhen the liquid is expressed from a particle, it immediately absorbsfresh liquid somewhat like the action of some conventional washingmachines.

Our novel process is attended by numerous advantages, some of which havebeen previously mentioned. In addition, we have found that in the caseof straw preliminary chopping and the like as a separate step prior tocooking may be eliminated, for whole straw may be fed into the hotliquor and transformed into pulp under our impact action almost asreadily as pro-chopped straw. The straws are sutficiently tendered sothat they are shredded easily by the action of the submerged impacting.

The nodes and rachises in straw have always been troublesome to thepulpmaker. They do not cook as readily as the culm or stem. The usualpulp beating or refining methods break down these relatively raw nodesand rachises to small particles which are not easily bleached and showup as specks in the final paper, and to the ultimate platelets and cellswhich slow down paper machine operation. rachises are swollen andsoftened, but not disintegrated, in contrast to results obtained withheaters or conventional refining engines. When our pulping operation iscompleted these nodes and rachises are substantially free of clingingfibers and can be removed by riflling and screening. Moreover, grainseeds which are normally digested and disintegrated by conventionalpulping and refining methods are retained on the riffier and the screen,in a swollen and softened condition. The pulp from our process thus hasa somewhat lower lignin content, better bleachability, and fewer specksthan a similar pulp defibered in a beater or disc refiner beforerifiling and screening.

Our mechano-chemical pulping may be accomplished by means of a widevariety of mechanisms capable of producing impact upon the freesubmerged material pieces, i e., immediately free to move upon beingstruck a vigorous blow. For convenience, we employ mechanical devicesdesigned for repulping paper, but our invention is not limited thereto.

One of the most important advantages of our process is our ability toproduce a satisfactory pulp at atmospheric pressure. This increases thesafety and reduces the cost of the process. It also increases the easeof control, since sampling can be done easily and rapidly and the In ourprocess the nodes and necessary processing controls accomplished moreefficiently. The lower temperatures result in a stronger pulp, since itis known that temperatures substantially above 100 C. tend to decreasethe length of the cellulose molecular chains, resulting in shorterchains and lower strength. Probably also for the same reasons, higheryields of pulp are obtained by our process. Our pulp also contains alarger proportion of hemicelluloses, such as pentosans, than are presentin the original straw. This is due to the mild essentiallynon-hydrolytic action of the cooking liquor at the lower temperatures.This process comes closer to the ideal pulping process, of removing thelignin with little or no attack on the carbohydrate portion of the strawor other fiber material, than any of the conventional pressure pulpingmethods.

The temperature of our process may vary within the range of 90 C. to 100C., preferably 96 C. to 98 C. Lower temperatures may be used, but theaction is slower. Slightly higher temperatures can be effected, but aregenerally unnecessary and, unless measures are taken to confine thevapors, result in excessive steam consumption.

Practically complete defibering, i. e., the separation of the fibersfrom the nodes and rachises as well as the breaking up of the fiberbundles into the individual fibers is accomplished by ourmechano-chemical process. We usually prefer to stop the process as soonas the straw is sufliciently pulped even though a small amount of fibermay still be attached to the nodes and rachises. This affords theadvantage of a high-yield, high-quality pulp and avoids any effect ofovercooking. If the material is not completely defibered within thepulping period, the final and complete defibering can be accomplished inthe same vessel after removal of the pulping chemical, or in any of thewell-known equipment used for defibering purposes.

In making fine pulps, equipment for this final defibering should beselected such that the nodes and rachises are not broken up in thedefibering process. For example, a second treatment in themechano-chemical vessel using water alone will accomplish this purpose,for the nodes and rachises will not be broken up or disintegrated, andit is easy to remove them from the pulp. Other defibering equipmentwhich does not break up or cut the nodes and rachises may be used. Inthe production of coarser pulps, for example, in 9-point corrugatingstrawboard, where broken nodes and rachises are not necessarilydisadvantageous, disc mills, heaters, jordan engines and the like may beused.

The invention is illustrated by the following experiments. In theseexperiments are described the preparation of strawboard pulp and finepulp from wheat straw, but our invention is not limited thereto, sinceit is applicable to any non-woody material, such as straw, flax or hemptow, cotton, sugar-cane bagassee, cornstalks, jute, reeds, manila andsisal fibers, and the like, and to coarse fiber material produced fromwood, such as Asplund fiber. McMillan fiber, fiber produced from woodchips in a disc mill, coarse groundwood fiber, chopped fine excelsiorand the like. Moreover, the invention is not to be understood as limitedby the particular chemical cooking liquor employed, for our invention isbased upon the mechanically induced diffusion of the chemical into andout of the material.

' The pulping was carried out in a device consisting of a vessel fittedat the bottom with a rotating plate to which were affixed a plurality ofblades so positioned that submerged particles of straw were given avigorous blow as the plate rotated rapidly. Rotation was started afterthe addition of the pulping liquor and the straw was added as rapidly aspossible. Direct steam was injected to maintain the temperature, and theperiod for combined digestion and defibering was measured from the timethe last of the straw was added. Samples were removed frequently fromthe vessel during the period and examined visually for degree ofpulping. At the end of the cooking period the pulped straw was removedfrom the vessel and washed. The fine pulp suitable for bleached paperand board products was screened before it was tested for strengthcharacteristics. Fine straw pulp and strawboard pulp were also preparedby conventional pressure pulping methods to serve as materials forcomparative purposes. The tests reported herein were carried out inaccordance with TAPPI standards.

In pulping straw for strawboard, the yields of washed pulp produced byour process, requiring one hours pulping time at 90 to 98 C. were 78 to79 percent, as compared with 75 to 78 percent for strawboard pulpproduced by cooking for 5 hours at 140 C. p. s. i.) using the sameamounts of pulping chemical in both types of cooks (6 percent lime and1.5 percent caustic soda, based on the dry straw). The pH of the blackliquor from our process was 11.1 as compared with 8.8 for that of theblack liquor from the pressure cook. This indicates less completeexhaustion of the active chemicals in our process.

The pulp made in accordance with our invention had better beating andstrength characteristics, particularly bursting strength and tearresistance, than the pressurecooked pulps. The comparative values forthe mechanochemical and pressure-cooked pulps, respectively, were:

Decreasing the amount of kraft chemicals used for cooking resulted inincreasing the time required to produce a pulp. The ash contents rangingfrom 1.9 to 3.7 percent with decreasing pulping chemical were lower thanthe ash of the pressure-cooked pulp (4.9 percent). The concentration ofcooking chemical in our process had practically no efiect on thepentosan content of the, pulps, which were very high, ranging from 32.5to 33.5 percent. This emphasizes the fact that our process ,approachesthe ideal in pulpingdelignification with relatively little action on thecarbohydrate portion of the plant material.

Since the cooks of our process are carried out at lower consistenciesthan those used in pressure cooking, thev concentration of organicmaterials in the blackliquors is relatively low. -A series of four cookswere made, using 10 percent kraft chemicals for the first cook. Thedrained liquor from the first cook was re-used in the second cook, morechemical being added to bring the total to 10 percent of the straw. Thisprocedure was repeated twice more. The chemical characteristics of thesepulps were quite similar except for the ash content which increased from1.6 to 2.5 percent with repeated re-use of the waste liquor. Re-use ofthe black liquor Table l Bursting Tear Freeness Tensile Crush T e ofPulp ii g (Schopperggffig strength, :53;? resistance Apparent ypRiegler), ream g./ream g'lreal'n (Riehle), density,

ml. pound pound pound pounds Mechano-chemical 340 66 131 58 46 0.76Pressure-cooked 45 345 60 122 47 44 o. 62

The somewhat higher apparent density of our pulps indicates betterdelignification, as compared with the pressure-cooked pulps, andconsequently better hydrating and felting characteristics of theresulting fibers. This is corroborated by the chemical analysis of ourpulps with 15.6 percent lignin and 30.4 percent pentosans as comparedwith 17.7 percent lignin and 25.5 percent pentosans for thepressure-cooked pulps. The color of the strawboard pulp was much lighterthan that of the pressure-cooked pulp.

For producing fine pulp, runs were made in accordance with our processusing as pulping reagents sodium sulfite and the usual kraft chemicals,sodium hydroxide plus sodium sulfide.

The results with strong pulping agents, such as the caustic soda andsodium sulfide of kraft liquor were better than those obtained withneutral sulfite. The neutral sulfite required a considerably longer timeto produce pulp from straw. With 12 percent kraft chemicals, based ondry straw, the yield of screened pulp obtained by digesting straw forone-half hour at 98 C. in accordance with our invention wasapproximately percent, about 8 percent greater than that obtained bycooking for two hours at 170 C. (100 p. s. i.) with the same amounts ofchemicals. The beating and strength characteristics of themechano-chemical and pressure-cooked unbleached kraft pulps,respectively, were very similar;

had little or no eifect on the yields or strength properties of thepulps.

We claim:

1. A mechano-chemical process for producing pulp comprising subjectingan undefibered non-woody fiber material to the action of a pulpingliquor containing a chemical reagent in solution in a liquid medium,said reagent being capable of digesting said material by removing ligninand other encrustants therefrom, at a temperature of about from to C andat about atmospheric pressure, said pulping liquor being essentiallynonhydrolytic under said conditions of temperature and pressure, whilesimultaneously subjecting the individual pieces of said material tovigorous non-cutting impact blows while submerged in the pulping liquorand while they are in a relatively free state, said non-cutting impactblows comprising mechanically-induced cycles of expression andabsorption, eachcycle comprising compacting the pieces in an unconfinedzone of impact to express liquid, immediately removing said pieces fromthe zone of impact to a zone of absorption of fresh liquid and returningsaid pieces containing absorbed liquid into another zone of impact, andcontinuing the said blows for from 30 to 60 minutes at which time asubstantial proportion of lignin has been removed from said pieces andthey have become broken up into individual fibers producing a pulp Thekraft pulp produced according to our invention had a much lighter colorthan that of the usual pressurecooked kraft straw pulp.

wherein the proportion of hemicelluloses is at least as great as thatcontained in the original non-woody fiber 75 material.

2. The method of claim 1 in which the undefibered 1,509,034 non-woodymaterial is straw. 1,549,103 3. The method of claim 1 in which theundefibered 1,654,624 non-woody material is bagasse. 1,770,430 4; Themethod of claim 1 in which the undefibered 5 1,785,840 non-woodymaterial is straw, and wherein the straw pulp 1,894,577 produced issubjected to a riffiing and screening operation 1,913,607 whereby nodes,rachises, seeds, and similar material are 1,914,184 removed therefrom.1,951,167 10 2,139,274 References Cited in the file of this patent 2 237332 UNITED STATES PATENTS 2,452,135

25,418 Keen Sept. 13, 1859 788,633 Cushing May 2, 1905 1 889,241 KennedyJune 2, 1908 1,015,S03 Judson Jan. 30, 191.2 1,039,941 Herz Oct. 1, 19128 Thornley et al. Sept. 16, Barnes Aug. 11, Wells Jan. 3, Respess July15, Munroe Dec. 23, Wells Jan. 17, McMillan June 13, Traquair June 13,Respess et a1. Mar. 13, Justice et a1. Dec. 6, Steely June 23, Lowe Oct.26,

OTHER REFERENCES 5 Jayme: Pap. Ind. and Pap. World, June 1946,

Sohn: Pap. Ind. and Pap. World, July 1943, page 444.

1. A MECHANO-CHEMICAL PROCESS FOR PRODUCING PULP COMPRISING SUBJECTINGAN UNDEFIBERED NON-WOODY FIBER MATERIAL TO THE ACTION OF A PULPINGLIQUOR CONTAINING A CHEMICAL REAGENT IN SOLUTION IN A LIQUID MEDIUM,SAID REAGENT BEING CAPABLE OF DIGESTING SAID MATERIAL BY REMOVING LIGNINAND OTHER ENCRUSTANTS THEREFROM, AT A TEMPERATURE OF ABOUT FROM 90* TO100* C AND AT ABOUT ATMOSPHERIC PRESSURE, SAID PULPING LIQUOR BEINGESSENTIALLY NONHYDROLYTIC UNDER SAID CONDITIONS OF TEMPERATURE ANDPRESSURE, WHILE SIMULTANEOUSLY SUBJECTING THE INDIVIDUAL PIECES OF SAIDMATERIAL TO VIGOROUS NON-CUTTING IMPACT BELOWS WHILE SUBMERGED IN THEPULPING LIQUOR AND WHILE THEY ARE IN A RELATIVELY FREE STATE, SAIDNON-CUTTING IMPACT BLOWS COMPRISING MECHANICALLY-INDUCED CYCLES OFEXPRESSION AND ABSORPTION, EACH CYCLE COMPRISING COMPACTING THE PIECESIN AN UNCONFINED ZONE OF IMPACT TO EXPRESS LIQUID, IMMEDIATELY REMOVINGSAID PIECES FROM THE ZONE OF IMPACT TO A ZONE ABSORPTION OF FRESH LIQUIDAND RETURNING SAID PIECES CONTAINING ABSORBED LIQUID INTO ANOTHER ZONEOF IMPACT, AND CONTINUING THE SAID BLOWS FOR FROM 30 TO 60 MINUTES ATWHICH TIME A SUBSTANTIAL PROPORTION OF LIGNIN HAS BEEN REMOVED FROM SAIDPIECES AND THEY HAVE BECOME BROKEN UP INTO INDIVIDUAL FIBERS PRODUCING APULP WHEREIN THE PROPORTION OF HEMICELLULOSES IS AT LEAST AS GREAT ASTHAT CONTAINED IN THE ORIGINAL NON-WOODY FIBER MATERIAL.